Methods for mathematically basic and precise description of system behavior at discrete interfaces have been developed by David Parnas and his groups and collaborators over many years. Total functions can play a crucial role as constructive and effectively executable semantics for various levels of these descriptions. Straightforward analysis and transformation techniques for functional programs, particularly effective for total functions, can be used as significant steps towards automated generation of implementations. Theoretical claims are supported by practical examples. The focus is on insight into applications from the functional perspective rather than on innovations in functional programming itself.